I've been using a Nexus One as my primary phone for the last few months now, and I quite like it! Upon reflection, the thing I most like about it, however, is not something I had anticipated at all when I got the phone.
Being able to search the web, read email, run apps, look at maps, and listen to podcasts are all very convenient. Though, there is not much to say about those things: I use those tools in more or less the same way I would use them if I had a laptop with me.
However, being able to send and receive IMs from my phone has been one of a small number of applications that has led to a qualitative change in the way I do things. I'm using IM a lot more now, at the expense of pretty much every other form of communication. And that's because instant messaging is the only mode of communication that actually feels convenient.
You can write and reply whenever you want, not just when it's convenient for the other person (unlike phone calls). You can have rapid back-and-forth conversations (unlike e-mail and voice mail). You can write messages of whatever length you want (unlike SMS). You can communicate unobtrusively, e.g. in a library (unlike phone calls and voice mail). You can read messages without going through an absurd interface (unlike voice mail). Your messages reach you on whatever device you're using— desktop, laptop, or phone (unlike phone calls and SMS). It doesn't cost an exorbitant amount of money (unlike SMS). (As for video calling, it has pretty much all the disadvantages and restrictions of voice calls, plus some more. It's a cool tech demo, but not something I expect to use on a day-to-day basis.)
If you think about where and when and how you can use each mode of communication, IM matches or surpasses pretty much everything else on most axes. Sometimes you need the phone or email for a high-bandwidth conversation or to deliver a large payload. But those occasions are getting to be few and far between.
Part of it is just the medium— IM is as synchronous or as asynchronous as you want it to be— but a good part of the goodness here is thanks to Android's implementation. You can dictate messages with your voice; for short common messages (e.g. "call me when you get home") it works quite well, so you can dash off quick messages using whatever modality is more convenient, keyboard or voice. And the Android notification system notifies you of new messages and lets you bring up conversations easily— but discreetly, without interrupting whatever else you are doing on the phone.
For me, IM is the killer app for carrying a smartphone in my pocket, ranking significantly above "browsing the web" and a fair amount above "making phone calls".
(Incidentally, the words "phone" and "smartphone" seem so inadequate after you have come to fathom this super-communications capability you have on your hands.)